1. Field of the Invention
The present invention relates to a radio communication system for communications between a plurality of radio stations, and also to an apparatus, method, and computer program for said radio communication system. More particularly, the present invention relates to a radio communication system having a network constructed under control of a specific control station and also to an apparatus, method, and computer program for radio communication for transmission of information within such a wireless network.
To be more specific, the present invention relates to a radio communication system in which there exist a plurality of networks which compete with one another and also to an apparatus, method, and computer program for such a radio communication system. More particularly, the present invention relates to a radio communication system to solve competition among a plurality of wireless networks which use different signal systems and also to an apparatus, method, and computer program for such a radio communication system.
2. Description of the Related Art
By constructing a local area network (LAN) from a plurality of computers connected to one another, it is possible to share information (such as file and data) and peripherals (such as printer) and to transmit and exchange information (such as e-mail and data contents).
It has been common practice to construct a LAN by wiring with optical fibers, coaxial cables, or twisted pair cables. This conventional LAN has the disadvantage of requiring troublesome wiring works, which prevents easy construction of a network. In addition, a wired LAN inconveniently limits the movement of apparatuses within the cable length. The wireless LAN, which has appeared as a new system to remove inconvenience from the conventional wired LAN, is now attracting attention. The wireless LAN of this kind eliminates most cables in an office and permits communication terminals, such as personal computers (PC), to be moved comparatively easily.
There is a rapidly increasing demand for the wireless LAN system as it achieves a greater speed and decreases in price. Nowadays, many users are considering the introduction of a personal area network (PAN), which is a small-scale wireless network for information exchange between a plurality of electronic apparatuses present around individual users. There are different radio communication systems based on specific frequency bands (such as 2.4 GHz and 5 GHz band) which do not need any license from supervisory offices.
The high-speed personal area network is being standardized in IEEE 802.15.3, which is based on the PHY (physical) layer utilizing signals in the 2.4 GHz band.
According to this IEEE 802.15.3, the MAC (medium access control) sublayer specified therein can be used as another PHY layer other than the PHY layer that uses signals in the 2.4 GHz band. The MAC sublayer has a contention access period (CAP) and a contention-free period (CFP). Asynchronous communications are performed by using the contention access period in which short data and command information are exchanged. On the other hand, stream communications are performed within the contention-free period in which slots called guaranteed time slot (GTS) are allocated for bandwidth reserve transmission.
Recently, standardization is going on to use the PHY layer specified in IEEE 802.15.3 for other PHY layers than those which use signals in the 2.4 GHz band.
Among systems for the high-speed wireless personal area network is UWB (ultra-wide-band). This radio communication system is designed to realize high-speed data transmission in which data is spread over an extremely wide frequency band, say, 2 to 6 GHz.
The UWB radio communication system sends a signal by repeating an impulse string, which has a prescribed pulse width, within a prescribed pulse period. The signal transmitted in this way is a signal string constructed of information bits. There are several possible ways of representing the signal string. In other words, one signal string may have a varied frequency band and a spread signal band width.
This signal string may be modulated in various ways. For example, it is possible to express the bit values of 0 and 1 by changing the phase of the pulse, and it is also possible to express multiple values by subtly changing the position of the pulse.
In the recent communication environment in which information equipment has come into general use, a variety of apparatuses are present in an office, and apparatuses are connected to one another through a wireless network, there occurs an undesirable instance in which wireless networks become congested in a limited area and a plurality of wireless networks use the same frequency band.
In such a case there is no means to exclude signals coming from another wireless networks. It is only permissible for one wireless network to begin transmission after confirming a period of time in which another wireless network remains idle. For example, the control station of a network decodes the beacon information coming from another network to obtain the band allocation information, then, based on such information, it excludes the band allocation region being used by another network and re-establishes the band allocation region to be used by its own network.
In the case of the above-mentioned UWB wireless communication network, data to be transmitted is spread over an extremely wide band width. This leads to a great possibility that one wireless communication network competes with its adjacent ones.
Because of its inherent properties, the UWB radio communication system employs signals which have been spread over the entire wide frequency band. This poses difficulties in operating the system by switching the frequency as in the conventional radio system.
In addition, the UWB radio communication system may use various signal strings and modulation schemes (which are collectively referred to as “signal system” hereinafter) depending on the signal aspect, impulse period, impulse width, spread ratio, and spreading code. The problem with the UWB radio communication networks which employ different signal systems is that signals from one network to another cause interference and the beacon signal from another network cannot be decoded because of non-interchangeability of signal expressions. The consequence is that it is impossible to mutually confirm the presence of networks or it is impossible to detect the region which is being used for communication by another network.